1. Field of the Invention
This invention relates to an apparatus for the manufacture of a continuous sheet of flat glass by supporting molten glass on a pool of molten metal while forming and cooling the glass. More particularly, this invention relates to an apparatus and method for delivering molten glass onto such a pool of molten metal for forming.
2. Description of the Prior Art
Molten glass may be delivered onto molten metal and formed into a continuous sheet or ribbon of glass according to the teachings of Heal, U.S. Pat. No. 710,357 or of Hitchcock, U.S. Pat. No. 789,911; or according to the teachings of Pilkington, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816; or according to the teachings of Edge and Kunkle, U.S. Pat. No. 3,843,346. According to the teachings of Heal, Hitchcock and Edge and Kunkle, molten glass is delivered over a rigid element and directly onto a glass-supporting pool of molten metal, while according to the teachings of Pilkington, molten glass is delivered over a rigid element and then allowed to fall freely onto a glass-supporting pool of molten metal where it spreads rearwardly and outwardly along the surface of the pool of molten metal. The rigid element which supports glass during its delivery is generally a refractory element. In the device of Hitchcock, it is merely an upper part of a separating wall. According to the teaching of Heal, it is a refractory bridge. According to the teachings of Pilkington, the refractory support is a refractory canal terminating in a refractory lip suspended above and overlying a pool of molten metal. According to the patent of Edge and Kunkle, glass is supported by a refractory threshold which has a glass-supporting surface extending into contact with the pool of molten metal onto which glass is to be delivered.
According to the patent of Pilkington, the glass which contacts the refractory piece upon which it is supported during delivery may be contaminated by that refractory piece due apparently to either erosion or corrosion of the refractory piece by the hot molten glass which is moving over it. The invention of Pilkington is directed toward minimizing bottom surface defects characterized as linear defects oriented along the direction of glass movement by providing a molten glass flow regime which permits the bottom, refractory-contacting portion of the delivered layer or stream of glass to flow rearwardly then outwardly into the marginal portions of the advancing layer of glass which is ultimately cooled and attenuated to form a continuous ribbon or sheet of flat glass.
According to U.S. Pat. No. 3,884,665 of Edge and Kunkle, molten glass may be delivered directly over a refractory threshold without creating undesirable bottom defects in glass. This is possible if the refractory threshold is provided with an upper glass-supporting surface that is convex in shape and particularly if the threshold is made of materials such as substantially pure silica or alumina. Further, U.S. Pat. No. 3,884,665 teaches that refractory markings may be further minimized by appropriate thermal control of a threshold over which molten glass is delivered for forming.
It is recognized that not all defects that are found at or near the bottom surface of a sheet or ribbon of glass are caused by reaction with or contamination by a refractory over which glass is delivered immediately prior to being delivered onto and supported by a pool of molten metal upon which it is formed. It is possible to have defects which derive from sources within a glassmaking furnace. A typical defect source may be a collection or buildup of devitrified glass or other contaminating material in a furnace, particularly in a portion of a furnace immediately upstream of a threshold or other delivery facility. It is further recognized that gases may be caused to transpire through relatively porous refractories into molten glass due to a thermal transpiration effect so that small seeds or bubbles of gas may be present in freshly delivered molten glass. Such seeds or bubbles would likely be in the lowermost portion of a layer of molten glass being delivered over a surface of a refractory member such as a threshold onto molten metal for forming. Bubbles of gas may have insufficient free energy to free themselves from such a surface and drift upwardly through the molten glass prior to its delivery from a refractory piece. Nevertheless, the moving layer of glass may provide sufficient energy to drag the bubbles from the surface of the refractory and carry them along with it. Since glass cools relatively rapidly immediately upon its delivery onto a pool of molten metal for forming, any bubbles of gas which enter the glass may not rise sufficiently fast to move through or into the body of glass itself but may exist near and at the bottom surface of the glass. They could then be evident as open bubbles in the bottom surface of the finished glass having been held in that position due to the rapidly increasing viscosity of the glass itself.
The present invention contemplates a modification to a molten glass delivery facility in order to prevent bubbles or seeds of gas or other contaminants from affecting and detracting from the overall quality of a major portion of the bottom surface of a sheet or ribbon of glass produced by flotation on molten metal following its delivery thereto by a direct delivery technique such as delivery over a threshold.